Plasma doping (“PLAD”), also known as Plasma Immersion Ion Implantation (“PIII”), processes are known in the art and are used to implant impurities into a semiconductor substrate. The semiconductor substrate is placed on a cathode within a plasma chamber and a doping gas including the desired impurity to be implanted is introduced into the plasma chamber. Positive ions from the doping gas are accelerated towards the semiconductor substrate and include impurity-based ion species, as well as non-impurity-based ion species. As used herein, the term “impurity-based ion species” means and includes ionic species in a plasma that include at least one atom of the impurity to be implanted in the semiconductor substrate and the term “non-impurity-based ion species” means and includes ionic species in the plasma that lack, or do not include, at least one atom of the impurity. As such, the impurity-based ion species account for a portion of the total ion species present in the plasma and implanted in the semiconductor substrate.
Since multiple ion species are produced and implanted in the semiconductor substrate, determining the dopant dose (“dosimetry”) or total impurity dose is a challenge with PLAD processes. As used herein, the phrase “total impurity dose” refers to the number of dopant atoms implanted per unit area of the semiconductor substrate and is reported as the number of implanted impurity atoms/cm2 of the semiconductor substrate. One dosimetry approach has been to monitor a total ion dose with a Faraday cup and then determine the total impurity dose by an ex situ or trial and error method. As used herein, the term “total ion” means and includes the combination of impurity-based ion species and non-impurity-based ion species and the term “total ion dose” means and includes the total number of impurity-based ions and non-impurity-based ions implanted per unit area of the semiconductor substrate. Depending on the doping gas used, multiple impurity-based ion species and/or multiple non-impurity-based ion species may be present in the plasma. One trial and error method involves using Secondary Ion Mass Spectroscopy (“SIMS”) to determine the total impurity dose and profile in the semiconductor substrate. The SIMS is conducted after the PLAD process is complete. The total ion dose determined with the Faraday cup in used in conjunction with the SIMS measurements to determine the total impurity dose. Another trial and error method utilizes four point probe resistance measurements and Spreading Resistance Profiling (“SRP”) measurements to determine the total impurity dose and profile and is conducted after the PLAD process and a post-implant annealing activation process. Both of these trial and error methods are undesirable because the semiconductor substrates are broken or cleaved to determine the total impurity dose. These methods are also undesirable because the total impurity dose is not determined until after the PLAD process is complete. As such, if the tested semiconductor substrate does not include the desired total impurity dose, the batch of semiconductor substrates subjected to the same implant conditions as the tested semiconductor substrate is discarded. The implantation conditions are then changed on a subsequent batch of semiconductor substrates, these semiconductor substrates are tested, and the process repeated until the implantation conditions produce the desired total impurity dose in the semiconductor substrate. The iterative nature of the trial and error methods is time consuming and wasteful because semiconductor substrates are destroyed in order to determine the total impurity dose. Additionally, these methods suffer from poor accuracy, controllability, and repeatability.
Therefore, it would be desirable to develop a method and an apparatus to determine impurity dosage of a semiconductor substrate during an implantation process, providing the capability of real-time process control.